Recently, liquid crystal display panels have been widely used as display units in notebook personal computers which are easy to carry. Such a liquid crystal display panel incorporates a cold-cathode fluorescent lamp serving as a backlight for backlighting the panel. In order to light this cold-cathode fluorescent lamp, a high voltage of about 1 kV or more is required. To maintain emission of light, a voltage of about several hundred volts must be applied.
In such notebook personal computers and similar products, owing to their characteristics, the demand for compact inverters for backlights is high. In order to meet this demand, piezoelectric transformers are used as inverter components.
As such a piezoelectric transformer, the piezoelectric transformer device (to be referred to as the piezoelectric device hereinafter) having the multilayer structure proposed in Japanese Patent Application No. 8-52553 filed by the present applicant will be briefly described below with reference to FIGS. 1 to 5.
FIG. 1 is a perspective view showing the piezoelectric device having the multilayer structure previously proposed by the present applicant. FIG. 2 is a front view of the piezoelectric device in FIG. 1. FIG. 3 is a plan view of the piezoelectric device in FIG. 1.
A piezoelectric device 106 has an outer shape like the one shown in FIGS. 1, 2, and 3. The right and left half regions of the device will be respectively referred to as the second and first regions hereinafter.
Outer electrodes 101 and 102 as primary (input) electrodes are formed on the upper surface of the first region. An outer electrode 103 as a secondary (output) electrode is formed on the right end portion of the second region. Lead wires 105 are connected to the outer electrodes 101, 102, and 103 with solder portions 104.
The internal structure of the first region will be described next. FIG. 4 is a sectional view taken along a line I--I of the piezoelectric device in FIG. 3. FIG. 5 is a sectional view taken along a line II--II of the piezoelectric device in FIG. 3.
As shown in FIGS. 4 and 5, in the first region, a plurality of inner electrodes 101a and 102a are alternately stacked, and the spaces between these inner electrodes are filled with the piezoelectric element 106. As shown in FIGS. 4 and 5, in the first region, the plurality of inner electrodes 101a are connected to each other through a columnar conductor (to be referred to as an interlevel connection conductor hereinafter) 108a, and the plurality of inner electrodes 102a are connected to each other through an interlevel connection conductor 108b. Circular holes (to be referred to as holes hereinafter) are formed in the inner electrodes 101a and 102a so that the inner electrodes 101a and 102a are not connected to each other through the interlevel connection conductors 108a and 108b.
The interlevel connection conductor 108a extends toward the upper surface of the first region and is connected to the outer electrode 101.
Similarly, the interlevel connection conductor 108b extends toward the upper surface of the first region and is connected to the outer electrode 102.
The piezoelectric device having the above multilayer structure is driven as follows. First, a high voltage is applied between the outer electrodes 101 and 102 to polarize the first region in the vertical direction (direction of thickness). A predetermined voltage is then applied between the outer electrode 101 or 102 and the outer electrode 103 to polarize the second region in the longitudinal direction. When an AC voltage is applied between the outer electrodes 101 and 102 of the device in the polarized state, the piezoelectric element 106 in the first region mechanically vibrates in accordance with the piezoelectric material constant unique to the piezoelectric element, the resonance characteristics, and the dimensions of the overall device. The vibrations are converted into a voltage by the piezoelectric element 106 in the second region. As a result, a boosted high AC voltage can be extracted from the outer electrode 103.
When such a piezoelectric transformer is to be mounted on a circuit board, the transformer is generally housed in an insulating container (to be referred to as a housing hereinafter), and the housing is mounted on the circuit board for the following reason. As described above, when the piezoelectric transformer operates, the piezoelectric device itself mechanically vibrates. If this vibration is inhibited when the device is mounted on the circuit board, the output characteristics of the piezoelectric transformer deteriorate. Therefore, when the piezoelectric transformer is to be mounted, the device must be supported at portions corresponding to nodes of the vibrations of the piezoelectric device to prevent the vibrations of the device from being suppressed. In addition, since the secondary output of the piezoelectric transformer is set at a high voltage, the entire piezoelectric transformer must be covered with an insulating cover for safety reasons, i.e., preventing contact between the secondary output and other components and preventing the operator from receiving an electric shock.
An example of how the above piezoelectric device is housed in a housing having a general structure will be described with reference to FIGS. 6 to 10.
FIG. 6 is a perspective view showing a state in which the piezoelectric device having the multilayer structure previously proposed by the present applicant is housed in a general housing. FIG. 7 is a plan view of the housing in FIG. 6.
As shown in FIGS. 6 and 7, a housing 107 is a box-like resin container having an upper opening. The housing 107 has a volume enough to prevent the piezoelectric device 106 and the solder portions 104 of the lead wires 105 from protruding from the upper opening of the housing 107 when the piezoelectric device 106 is housed in the housing 107. Projections 110 for fixing the piezoelectric device 106 in the housing 107 are formed on the inside of the housing 107 at positions corresponding to the nodes of the vibrations of the piezoelectric device 106. In the case shown in FIGS. 6 and 7, the respective lead wires 105 extend outside through the holes (the holes may be slits) formed in the end faces in advance.
A method of housing the piezoelectric transformer in the housing 107 in this housed state will be described below, together with the mounted state of the housing on a circuit board.
FIG. 8 is a sectional view taken along a line III--III of the housing in FIG. 6. FIG. 9 is a sectional view taken along a line IV--IV of the housing in FIG. 6. FIG. 10 is a perspective view showing an example of how the piezoelectric transformer is mounted on the circuit board.
When the piezoelectric device 106 is to be housed in the housing 107, the projections 110 are first coated with an adhesive 109. As shown in FIGS. 8 and 9, the piezoelectric device 106 is then housed in the housing 107 such that the nodes of the vibrations of the piezoelectric device 106 coincide with the projections 110. With this process, the piezoelectric can be supported in the housing 107 at the portions corresponding to the nodes of vibrations.
The housing is mounted on the circuit board as follows. After the adhesive 109 applied to the projections 110 is hardened, the housing 107 is turned upside down and mounted on a circuit board 112, as shown in FIG. 10. The housing 107 is fixed to the circuit board 112 with an adhesive 111. The respective lead wires 105 extending through the above holes are connected to the circuit board 112 by soldering.
Other methods of supporting the piezoelectric device and mounting it on the circuit board, other than those described above, have been proposed (Japanese Patent Laid-Open Nos. 8-32135 and 8-264853).
When the piezoelectric device is to be housed in the above general housing, a large number of steps are required, ranging from the step of housing the piezoelectric device in the housing to the step of connecting the lead wires on the circuit board, resulting in low productivity. When terminals to which the lead wires of the piezoelectric device are connected are to be formed on the housing, and the housing is to be mounted on the circuit board through the terminals, a large number of steps are required in mounting the housing, resulting in low productivity.
For example, Japanese Patent Laid-Open No. 8-298213 discloses a housing superior to the housing having the above structure in terms of productivity. According to this housing, connection of the piezoelectric device to the circuit board and supporting of the device in the housing are performed by using the same terminals, thereby obviating the necessity to use lead wires to connect the piezoelectric device to the circuit board. According to this technique, the piezoelectric device is supported in the housing by using only the elastic characteristics of the terminal members (material) formed on the housing. If, therefore, an external shock or vibration acts on this housing, the electric connection between the piezoelectric device and the terminals may be broken, resulting in instantaneously disconnection.